The wagon wheel method was developed to determine the ER under a mask
that is aligned along a particular crystallographic direction of a wafer of a given
crystallographic orientation. The idea is that all crystallographic orientations in the
zone of the wafer orientation are offered to the etchant, and one hoped to be able to
extract the rate at which the mask is underetched for all directions in the zone. For
example, to measure the ER of the (111) plane, we make an opening along the (110)
direction which is the intersection line of the (111) plane with wafer surface, and
measure the undercutting of the silicon substrate after a certain time in TMAH. In
order to get the ER of different orientations, a mask consisting of many spokes along
different orientations is designed, as shown in the following picture. This method is
the wagon wheel method